Carbon black is conventionally produced by the thermal decomposition of an oil. Hot combustion gases are first formed in a reactor by the burning of a fuel in the presence of a free oxygen containing gas. These gases are then used to heat and decompose the oil that is injected into the reactor. The decomposition of the oil forms carbon black. The carbon black so formed must be recovered from a residual stream of gases that are admixed with the carbon black. Prior to attempting to recover the carbon black, the reaction can be quenched by introducing a stream of water into the downstream region of the reactor.
Various techniques have been utilized to recover carbon black from the reaction off gas stream. For instance, filtration of the reaction off gas more or less separates the particles of carbon black from the gaseous effluent stream.
The carbon black particles, however, are very fine. Particle or aggregate size in the carbon black reactor effluent typically range from about 0.05 to about 1 microns. Any satisfactory filtration system must recover nearly all of the carbon black from the gas stream prior to release of the gas stream to the environment or use of the gas stream in energy recovery equipment.
In the prior art, various arrangements of filtration apparatus and filtration mediums have been employed to separate particles of carbon black from a carrying gas stream. Perforated or porous ceramic or metal sheets, beds of sand, wire mesh, etc., have been used alone or in combination with fabric filters. Fabric filters such as bag filters made from synthetic fibers, metal fibers, glass fibers and natural fibers such as cotton or wool are well known in the art.
The filtration medium allows the gas to pass through the filter but retains the particles of carbon black. The deposits of the particles create a filter cake which is a layer of particles of carbon black on the filtration medium. Typically the filter cake is nearly completely removed from the various filtration mediums by first taking the filter out of service and then adding a back flow of gas or by washing the filter with a back flush of water. The reverse flow of fluid dislodges almost the entire filter cake of carbon black particles. The filter is then put back in service, another filter cake is built from deposits of carbon black particles and eventually the reverse flow portion of the cycle used to dislodge the entire filter cake is repeated. It is important to note that filtering efficiency is drastically reduced prior to the build-up of new cake. Typically the particles of carbon black which form the filter cake are recovered by allowing them to fall from the filtration medium by gravity to a basin. The carbon black is removed from the basin by devices such as belts, mechanical conveyors, augers, pneumatic conveying, etc. The carbon black is subsequently directed to a recovery device, such as to a pelletizer. There it is admixed with a pelletizing agent such as water to form pellets. These pellets are then dried to remove the added water and are recovered as the final product.
Although there has been considerable improvement in the filtration and recovery of carbon black, there is still room for improvement as many serious problems are encountered in the use of prior art processes. For instance, with some prior art filters, the repeated back flush-filtration cycle decays and deteriorates the filtration medium. Bag filters for example have a useful life that is measured by the number and frequency of reverse or backflush flows.
With certain prior art filters, the carbon black particles penetrate the pores of the filtration medium in such manner that they cannot be easily removed by reverse flows of fluids. This results in an unacceptable pressure drop across the filter system. In prior filters having a filtration media of sand, for example, the pores formed by the spaces created between individual particles of sand form a labyrinth path which the particles of carbon black dust follow. The very fine long particles of carbon black penetrate the bed of sand along these relatively large channels that are formed between the individual particles of sand. The fine carbon black will eventually be entrapped in one of the smaller interstices, if the bed of sand is of sufficient depth. Particles of carbon black then build up within the sand and eventually must be removed to prevent an extremely large pressure drop across the sand filter. A reverse flow of a gas or liquid alone is not always successful in removing the entraped particles. Complete fluidization of the entire bed of particles of filtration medium is not always practical.